Laser fusion is a process by which a pellet of deuterium and tritium is heated by an intense beam of laser light to a temperature sufficiently high to initiate a fusion reaction. The temperature necessary to induce fusion is approximately 100,000,000.degree. C. Achievement of such high temperatures for even minute fractions of a second requires laser beams of extremely high power.
It has been found that laser pulses of high power may be obtained with the use of rare gas halide excimer laser amplifiers. The problems with these amplifiers is that they lack storage capability. Efficient energy extraction from such amplifiers requires long pulse operation, e.g., of the order of 500 ns.
One method of obtaining efficient energy extraction from the laser amplifier is to cause it to see a temporally contiguous train of short pulses simulating a single long pulse. In an encoder, these pulses are spatially overlapping but temporally contiguous and angularly dispersed in such a way that they spatially separate after propagating some distance. The pulses are then amplified and recombined with suitable time delays to form a single high energy composite pulse.
Various techniques are presently known for temporally and spatially encoding light pulses.
One such technique utilizes a series of beamsplitters each tilted at a different angle but aligned so that a short pulse passes through each beamsplitter. The reflected beams, however, propagate in various directions. These pulses can be individually amplified. Next, the pulses can be temporally synchronized and combined into a single pulse through appropriate delay means. Such an encoding system has the disadvantages that each pulse undergoes substantial attenuation from the original pulse and optical aberrations caused by each beamsplitter are cumulative. In addition, such a system requires a beamsplitter and an amplifier for each pulse which is to form part of the desired composite pulse.
Another technique utilizes a sequence of optical components arranged in a slightly misaligned ring. A master pulse is typically introduced into the ring at a beamsplitter. The misalignment causes a series of pulses to leave the ring at a linearly increasing field angle. Such an arrangement provides temporal and spatial encoding to facilitate recombination of the exiting pulses into a high energy composite pulse. To compensate for energy loss at the exit, which is a beamsplitter, an amplifier may be incorporated into the ring. This is particularly essential where the device is to be used in laser fusion.
The problem with the second foregoing described device is that the increasing field angle complicates beam cleanup by spatial filtering. Defocus due to aberrations such as Petzval curvature and astigmatism becomes progressively worse. Also, in order to accommodate the progressively increasing field angle, lenses in the system must have "fast" relative apertures which causes significant spherical aberration. Other aberrations are also present, the correction of which complicates and increases the number of optical elements necessary in the system.
The present invention overcomes or substantially alleviates the above discussed problems by providing an encoder that spatially separates beams by generating a series of temporally phased laser pulses as an evenly spaced conical array of pulsed beams at a constant field angle.